Hemodynamic stability assessment based on heart sounds

ABSTRACT

A system comprising an implantable medical device (IMD). The IMD includes an implantable sensor operable to produce an electrical signal representative of mechanical activity of a heart of a subject and a controller circuit coupled to the sensor. The IMD also includes a heart sound sensor interface circuit to produce a heart sound signal, a tachyarrhythmia detector, and a controller circuit. The controller circuit includes a hemodynamic stability assessment module configured to determine that at least one episode of ventricular tachyarrhythmia is detected in a subject and obtain a measurement of hemodynamic stability of the ventricular tachyarrhythmia from the heart sound signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending, commonlyassigned U.S. patent application Ser. No. 10/900,570 entitled“DETERMINING A PATIENT'S POSTURE FROM MECHANICAL VIBRATIONS OF THEHEART,” filed on Jul. 28, 2004, Ser. No. 10/703,175, entitled “A DUALUSE SENSOR FOR RATE RESPONSIVE PACING AND HEART SOUND MONITORING,” filedon Nov. 6, 2003, Ser. No. 10/334,694 entitled “METHOD AND APPARATUS FORMONITORING OF DIASTOLIC HEMODYNAMICS,” filed on Dec. 30, 2002, Ser. No.10/746,874 entitled “A THIRD HEART SOUND ACTIVITY INDEX FOR HEARTFAILURE MONITORING,” filed on Dec. 24, 2003, Ser. No. 11/037,275,entitled “METHOD FOR CORRECTION OF POSTURE DEPENDENCE ON HEART SOUNDS,”filed on Jan. 18, 2005, Ser. No. 11/129,050, entitled “METHOD ANDAPPARATUS FOR CARDIAC PROTECTION PACING,” filed on May 16, 2005, andSer. No. 11/148,107, entitled “ISCHEMIA DETECTION USING HEART SOUNDSENSOR,” filed on Jun. 8, 2005, each of which is hereby incorporated byreference.

TECHNICAL FIELD

The field generally relates to implantable medical devices and, inparticular, but not by way of limitation, to systems and methods formonitoring mechanical activity of the heart.

BACKGROUND

Implantable medical devices (IMDs) are devices designed to be implantedinto a patient. Some examples of these devices include cardiac functionmanagement (CFM) devices such as implantable pacemakers, implantablecardioverter defibrillators (ICDs), cardiac resynchronization devices,and devices that include a combination of such capabilities. The devicesare typically used to treat patients using electrical or other therapyand to aid a physician or caregiver in patient diagnosis throughinternal monitoring of a patient's condition. The devices may includeone or more electrodes in communication with sense amplifiers to monitorelectrical heart activity within a patient, and often include one ormore sensors to monitor one or more other internal patient parameters.Other examples of implantable medical devices include implantablediagnostic devices, implantable insulin pumps, devices implanted toadminister drugs to a patient, or implantable devices with neuralstimulation capability.

Additionally, some IMDs detect events by monitoring electrical heartactivity signals. In CFM devices, these events include heart chamberexpansions or contractions. By monitoring cardiac signals indicative ofexpansions or contractions, IMDs are able to detect tachyarrhythmia.IMDs are further able to provide therapy for tachyarrhythmia, such as ahigh energy shock stimulus or anti-tachyarrhythmia pacing (ATP).Tachyarrhythmia includes abnormally rapid heart rate, or tachycardia,including ventricular tachycardia (VT) and supraventricular tachycardia.Tachyarrhythmia also includes rapid and irregular heart rate, orfibrillation, including ventricular fibrillation (VF). Typically, ICDsdetect tachyarrhythmia by first detecting a rapid heart rate. Otherdetection methods in addition to fast rate detection are used to reducethe incidence of inappropriate shocks. The present inventors haverecognized a need for improved sensing of events related to devicetreatment of tachyarrhythmia.

SUMMARY

This document discusses, among other things, systems and methods formonitoring mechanical activity of the heart. A system example includesan implantable medical device (IMD). The IMD includes an implantablesensor operable to produce an electrical signal representative ofmechanical activity of a heart of a subject and a controller circuitcoupled to the sensor. The IMD also includes a heart sound sensorinterface circuit to produce a heart sound signal, a tachyarrhythmiadetector, and a controller circuit. The controller circuit includes ahemodynamic stability assessment module configured to detect at leastone episode of ventricular tachyarrhythmia in a subject and obtain ameasurement of hemodynamic stability of the ventricular tachyarrhythmiafrom the heart sound signal.

A method example includes detecting at least one episode of ventriculartachyarrhythmia in a subject, sensing at least one heart sound signalfor the subject using an IMD, and obtaining a measurement of hemodynamicstability of the ventricular tachyarrhythmia from the heart soundsignal.

This summary is intended to provide an overview of the subject matter ofthe present patent application. It is not intended to provide anexclusive or exhaustive explanation of the invention. The detaileddescription is included to provide further information about the subjectmatter of the present patent application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of portions of a system that uses animplantable medical device (IMD).

FIGS. 2A-B illustrate IMDs coupled by one or more leads to heart.

FIGS. 3A-B show an example of an IMD that does not use intravascularleads to sense cardiac signals.

FIG. 4 is a block diagram illustrating portions of an IMD used to obtainpatient hemodynamic information from heart sounds.

FIG. 5 shows a block diagram of portions of another example of an IMD.

FIG. 6 shows waveforms corresponding to cardiac cycles.

FIG. 7 shows a graph of a measurement of hemodynamic stability.

FIG. 8 is a graph of aortic pressure as a function of heart beats of asubject.

FIG. 9 shows a graph of a measurement of hemodynamic stability versusaortic pressure.

FIG. 10 is a block diagram illustrating portions of a system thatincludes an IMD used to obtain patient hemodynamic information fromheart sounds.

FIG. 11 is a block diagram of a method of monitoring mechanical activityof a heart.

FIG. 12 shows a block diagram of a method that extends an ATP therapyregimen that is provided by an IMD.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and specific embodimentsin which the invention may be practiced are shown by way ofillustration. It is to be understood that other embodiments may be usedand structural or logical changes may be made without departing from thescope of the present invention.

This document discusses systems and methods for improved detection ofcardiac events. A rapid and unstable heart rate associated withtachyarrhythmia can prevent the heart chambers from filling properly;resulting in a drop in a patient's blood pressure. Sometimes, a heartrate becomes rapid but a patient's hemodynamic system remains stable,i.e. the heart rate is regular enough so that the heart chambers areable to fill adequately to maintain adequate blood pressure. A properassessment of hemodynamic system stability is important in making adecision in whether to deliver or to delay treatment, or whether totreat a tachyarrhythmia with either shock or ATP therapy. However, it isdesirable that the decision be made in timely fashion.

Waiting too long to provide treatment may decrease the likelihood ofsuccessfully converting the rhythm once the chosen therapy is begun.Similarly, spending too much time trying various ATP regimens beforeresorting to shock therapy may also decrease the likelihood ofsuccessfully converting the tachyarrhythmia to a normal sinus rhythm.However, even a modest delay in providing treatment may allow anabnormal tachyarrhythmia rhythm to spontaneously revert back to normal.Delay of shock therapy may be warranted if it is known that thepatient's blood pressure remains adequate. If a device is able toproperly delay shock therapy, the number of inappropriate shocks isreduced, thereby enhancing patient comfort and extending the batterylife of the device.

Blood pressure remains adequate during a stable tachyarrhythmia butbecomes inadequately low during an unstable tachyarrhythmia. Makinginformation related to a patient's blood pressure within the heartchambers available to a device improves the chances that the device willmake a proper assessment of heart rhythm stability. A proper assessmentmakes it possible to delay the onset of treatment of a stabletachyarrhythmia or to extend the time to attempt to resolve the episodewith ATP before resorting to high-energy shock therapy. This ensuresthat a high-energy shock stimulus will convert the abnormal rhythm ifthe rhythm fails to convert spontaneously or fails to convert after ATP.

FIG. 1 is a block diagram of portions of a system 100 that uses animplantable medical device (IMD) 110. As an example, the system 100shown is used to treat a cardiac arrhythmia. The IMD 110 typicallyincludes an electronics unit that is typically coupled by a cardiac lead108, or additional leads, to a heart 105 of a patient 102, or otherwiseassociated with the heart 105. Examples of IMD 110 include, withoutlimitation, a pacer, a defibrillator, a cardiac resynchronizationtherapy (CRT) device, or a combination of such devices. System 100 alsotypically includes an IMD programmer or other external device 170 thatcommunicates wireless signals 160 with the IMD 110, such as by usingradio frequency (RF) or other telemetry signals.

Cardiac lead 108 includes a proximal end that is coupled to IMD 110 anda distal end, coupled by an electrode or electrodes to one or moreportions of a heart 105. The electrodes typically deliver cardioversion,defibrillation, pacing, or resynchronization therapy, or combinationsthereof to at least one chamber of the heart 105. The electronics unitof the IMD 110 typically includes components that are enclosed in ahermetically-sealed canister or “can.” Other electrodes may be locatedon the can, or on an insulating header extending from the can, or onother portions of IMD 110, such as for providing pacing energy,defibrillation energy, or both, in conjunction with the electrodesdisposed on or around a heart 105. The lead 108 or leads and electrodesmay also typically be used for sensing intrinsic or other electricalactivity of the heart 105.

FIGS. 2A-B illustrate IMDs 110 coupled by one or more leads 108A-C toheart 105. Heart 105 includes a right atrium 200A, a left atrium 200B, aright ventricle 205A, a left ventricle 205B, and a coronary sinus 220extending from right atrium 200A. In the example in FIG. 2A, atrial lead108A includes electrodes (electrical contacts, such as ring electrode225 and tip electrode 230) disposed in an atrium 200A of heart 105 forsensing signals, or delivering pacing therapy, or both, to the atrium200A.

Ventricular lead 108B includes one or more electrodes, such as tipelectrode 235 and ring electrode 240, for sensing signals, deliveringpacing therapy, or both sensing signals and delivering pacing therapy.Lead 108B optionally also includes additional electrodes, such as fordelivering atrial cardioversion, atrial defibrillation, ventricularcardioversion, ventricular defibrillation, or combinations thereof toheart 105. Such defibrillation electrodes typically have larger surfaceareas than pacing electrodes in order to handle the larger energiesinvolved in defibrillation. Lead 108B optionally providesresynchronization therapy to the heart 105.

In some examples, leads 108A and 108B are combined into one leadcontaining four electrodes located sequentially along the lead. In anexample, a first tip electrode is located in the apex of the rightventricle 205A, a first ring electrode located proximal to the tipelectrode and in the right ventricle 205A, a second ring electrodelocated proximal to the first ring electrode and in the right atrium200A, and a third ring electrode located proximal to the second ringelectrode and also located in the right atrium 200A.

The example in FIG. 2B includes a third cardiac lead 108C attached tothe IMD 110 through the header 255. The third lead 108C includes ringelectrodes 260 and 265 placed in a coronary vein lying epicardially onthe left ventricle (LV) 205B via the coronary vein 220. In the example,lead 108B further includes a first defibrillation coil electrode 275located proximal to tip and ring electrodes 235, 240 for placement in aright ventricle (RV), and a second defibrillation coil electrode 280 forplacement in the superior vena cava (SVC) located proximal to the firstdefibrillation coil 275, tip electrode 235, and ring electrode 240. Insome examples, high energy shock therapy is delivered from the first orRV coil 275 to the second or SVC coil 280. In some examples, the SVCcoil 280 is electrically tied to an electrode formed on the IMD can 250.This improves defibrillation by delivering current from the RV coil 275more uniformly over the ventricular myocardium. In some examples, thetherapy is delivered from the RV coil 275 only to the electrode formedon the IMD can 250.

Other forms of electrodes include meshes and patches which may beapplied to portions of heart 105 or which may be implanted in otherareas of the body to help “steer” electrical currents produced by IMD110. The present methods and systems will work in a variety ofconfigurations and with a variety of electrodes.

FIGS. 3A-B show an example of an IMD 300 that does not use intravascularleads to sense cardiac signals. FIG. 3A shows that the IMD 300 includesa thicker end 313 to hold the power source and circuits. The IMD 300also includes electrodes 325 and 327 for remote sensing of cardiacsignals. Cardioversion/defibrillation is provided through electrodes 315and 317. FIG. 3B shows an example of the position of the IMD 300 withina patient.

IMDs can include sensors to monitor heart sounds. An accelerometer isone type of heart sound sensor. An accelerometer converts anacceleration signal due to acoustic vibrations of a heart sound into anelectrical signal. A microphone is another type of heart sound sensor. Astrain gauge is yet another type of heart sound sensor. A strain gaugeconverts deformation of the sensor due to heart sound vibrations into anelectrical signal. Heart sounds include the “first heart sound” or S1,the “second heart sound” or S2, the “third heart sound” or S3, the“fourth heart sound” or S4, and their various sub-components. S1 isindicative of, among other things, mitral valve closure, tricuspid valveclosure, and aortic valve opening. S2 is indicative of, among otherthings, aortic valve closure and pulmonary valve closure. S3 is aventricular diastolic filling sound often indicative of certainpathological conditions including heart failure. S4 is a ventriculardiastolic filling sound resulted from atrial contraction and is usuallyindicative of pathological conditions. The term “heart sound” refers toany heart sound (e.g., S1) and any components thereof (e.g., M1component of S1, indicative of mitral valve closure). The term heartsound also includes audible and inaudible mechanical vibrations causedby cardiac activity that can be sensed with an accelerometer.

Monitoring heart sounds allows measurement of hemodynamic performanceparameters. Heart sounds signals obtained from heart sound sensors areindicative of timing, strength, and frequency characteristics related tothe heart sounds. Measuring these characteristics allows conclusions tobe made about the condition of a patient's hemodynamic system.

FIG. 4 is a block diagram 400 illustrating portions of an IMD 405 usedto obtain patient hemodynamic information from heart sounds. The IMD 405includes a an implantable heart sound sensor 410, a heart sound sensorinterface circuit 415, a tachyarrhythmia detector circuit 420, and acontroller circuit 425 that, in turn, includes a hemodynamic stabilityassessment module 430. The implantable heart sound produces anelectrical signal representative of at least one heart sound. The heartsound associated with mechanical activity of a patient's heart. In someembodiments, the implantable heart sound sensor 410 includes anaccelerometer. In some embodiments, the heart sound sensor 410 includesa strain gauge. In some embodiments, the heart sound sensor 410 includesa microphone.

The heart sound sensor interface circuit 415 produces a heart soundsignal. FIG. 5 shows a block diagram 500 of portions of an example of anIMD 505 where the heart sound sensor is an accelerometer 510 and theheart sound sensor interface circuit 515 includes an amplifier circuit517 and a filter circuit 518. In some examples, the amplifier circuit517 is a voltage amplifier that has a signal gain of about one thousandand the filter circuit 518 is a low-pass filter circuit having singlepole roll-off with a corner frequency between fifty hertz (50 Hz) toeighty hertz (80 Hz). In some examples, the controller circuit 525adjusts the parameters of the amplifier circuit 517 and filter circuit518. As illustrative examples, the controller circuit is configured toadjust the gain of the amplifier circuit 517 and to adjust the cornerfrequency of the filter circuit 518 or to change the filter circuit 518from single to double pole roll-off. In some examples, the heart soundsensor interface circuit 515 includes an analog-to-digital (A/D)converter circuit 519 to convert the analog heart sound signal fromaccelerometer 510 into digital values.

The energy content in heart sounds is predominantly contained in lowerfrequencies. In some examples, if the low-pass filter circuit 518 has asharp roll-off, the heart sound sensor interface circuit 515 includes apre-emphasis circuit. The pre-emphasis circuit has a transfer functionof a high-pass filter circuit, but with a lower order than the low-passfilter circuit. The pre-emphasis circuit adds signal gain to higherfrequencies of a low-pass filtered heart sound signal. These frequenciesinclude those near the corner frequency of the low-pass filter forexample. If the heart sound signal has been digitized, the pre-emphasistransfer function can be implemented mathematically by the controllercircuit 525. The term “controller circuit” includes a microcontroller, amicroprocessor, a digital signal processor, or application specificintegrated circuit (ASIC).

Returning to FIG. 4, the controller circuit 425 is configured to executea function or functions. Such functions correspond to modules, which aresoftware, hardware, firmware or any combination thereof. Multiplefunctions are performed in one or more modules. The controller circuit425 includes a hemodynamic stability assessment module 430. Thehemodynamic stability assessment module 430 can be configured to receiveinformation indicating that at least one episode of ventriculartachyarrhythmia in a subject has been detected by the tachyarrhythmiadetector circuit 420, and to obtain a measurement of hemodynamicstability of the ventricular tachyarrhythmia from the heart soundsignal.

In an illustrative example, the tachyarrhythmia detector circuit 420 iscoupled to a ventricular contraction sensing circuit and ventriculartachyarrhythmia is detected from a rapid ventricular contraction rate.If the measurement of hemodynamic stability indicates that thetachyarrhythmia is hemodynamically stable, treatment for thetachyarrhythmia can be delayed, or the time to attempt to resolve theepisode with ATP can be extended, before resorting to high-energy shocktherapy.

FIG. 6 shows waveforms 600 corresponding to sensed cardiac cycles. Theheart sound signal waveform 605 shows the relationship of the first,second, and third heart sounds (610, 615, 620) to the P-wave and the QRScardiac complex of an electrocardiogram 625. Because heart sounds areassociated with mechanical function of the heart, a measurement of ahemodynamic parameter based on heart sounds can be a proxy measurementof hemodynamic stability based on aortic pressure.

One method to obtain a measurement of hemodynamic stability is to uselinear prediction. Linear prediction is a statistical analysis tool thatcan be used to estimate future values of a sampled or discrete timefunction based on previous sampled values. In some examples, themeasurement of hemodynamic stability is a heart sound based hemodynamicparameter He which is defined asHe=1−k ₁ ²,  (1)where k₁ is a first reflection parameter. The first reflection parameterk₁ is a function of the first and second autocorrelation terms R₀ andR₁. Specifically,

$\begin{matrix}{k_{1} = {- {\left( \frac{R_{1}}{R_{0}} \right).}}} & (2)\end{matrix}$If a set of N digitized samples (s₀, s₁, s₂, s₃ . . . ) is taken overone measured heart sound, then the first and second autocorrelationterms R₀ and R₁ can be calculated as

$\begin{matrix}{{R_{0} = {\sum\limits_{n = 0}^{N - 1}s_{n}^{2}}},{and}} & (3) \\{R_{1} = {\sum\limits_{n = 0}^{N - 2}{s_{n}{s_{n + 1}.}}}} & (4)\end{matrix}$

In some examples, the hemodynamic parameter He is based on the S1 heartsound. FIG. 7 shows a graph 700 of He obtained from the S1 heart soundas a function of the ratio of the frequency of the measured S1 heartsound to the sampling frequency. The graph 700 shows that He will varywith frequency of the heart sound, which a measured with a fixedsampling rate.

FIG. 8 is a graph 800 of aortic pressure as a function of heart beats ofa subject. In the first region 805, a baseline of He can be established.In the second region 810, an episode of tachyarrhythmia was induced inthe subject. The third region 815 shows the change in aortic pressureduring the episode of tachyarrhythmia. After more than one hundred beatsin stable tachyarrhythmia in region 815, the episode became unstable inregion 820 and a large corresponding decrease in aortic pressure 822 isevident. The graph 800 shows defibrillation in region 825 followed byrecovery in region 830.

The hemodynamic parameter He was measured on a beat-by-beat basis forthe S1 heart sound during the episode. FIG. 9 shows a graph 900 of themeasured He versus aortic pressure. Region 905 shows a concentration ofmeasured He corresponding to the baseline 815 in FIG. 8. Region 915shows a concentration of measured He corresponding to the stabletachyarrhythmia of region 815 in FIG. 8. The graph 900 shows a decreasein both the measured He and the aortic pressure between regions 905 and915. FIG. 7 shows that the hemodynamic parameter He decreases if thefrequency components of the heart sound decrease. The decrease in thefrequency components of the S1 heart sound (evidenced by the decrease inthe hemodynamic parameter He) that occur with the decrease in aorticpressure may be due to the change in contractility of the heart duringtachyarrhythmia. Region 920 shows a concentration of measured Hecorresponding to the unstable tachyarrhythmia of region 820. Note thatwhile the graph 900 shows an overlap with some measured values of He,there is a large decrease in aortic pressure indicating that thetachyarrhythmia became unstable.

The graph 900 also shows that a threshold value T 925 of measured He canbe chosen so that if, during tachyarrhythmia, a value of He less thanthe threshold value 925 is measured, the patient's aortic blood pressuremay have become inadequately low. The tachyarrhythmia is deemed to beunstable due to the possibly low blood pressure and shock therapy shouldbe administered to the patient. If during tachyarrhythmia, a valuehigher than the threshold value 925 is measured, treatment can bedelayed or a regimen of ATP can be administered to the patient insteadof, or prior to, delivery of a shock. In another example, thetachyarrhythmia is deemed to be unstable if a value of He less than thethreshold value 925 is measured in X of Y consecutive cardiac cycles,where X and Y are integers and Y≧X. In one example, an appropriatethreshold value 925 of measured He can be determined from a databasecontaining sampled heart sound data and pressure data from severaltachyarrhythmia patients. In another example, an appropriate thresholdvalue 925 of measured He specific to one patient can be determined froma database containing such data only for that patient.

The graph 900 also shows that a baseline value of measured He can bedetermined for a patient when the patient is not experiencingtachyarrhythmia (region 905). In an example, the baseline is determinedfrom a central tendency of the measured He, such as mean value or amedian value. A patient's aortic blood pressure may have becomeinadequately low if the measured He changes from the baseline He valueby more than a predetermined threshold value. A corresponding detectedtachyarrhythmia is then deemed to be unstable. In another example, thetachyarrhythmia is deemed to be unstable if the measured He changes,such as a decrease, from the baseline He value by more than apredetermined threshold value in X of Y consecutive cardiac cycles,where X and Y are integers and Y≧X.

Returning to FIG. 4 and in light of the previous discussion, in someexamples, the heart sound sensor interface circuit 415 includes asampling circuit to obtain a sampled heart sound signal and thehemodynamic stability assessment module 430 includes an autocorrelationmodule to obtain the measurement of hemodynamic stability He bydetermining an autocorrelation function using the sampled heart soundsignal. The hemodynamic stability assessment module 430 deems theventricular tachyarrhythmia to be unstable when the measurement ofhemodynamic stability He is below a predetermined threshold value, suchas a fixed or programmable threshold value.

In some examples, the hemodynamic stability assessment module 430includes a baseline module to establish a baseline for the measurementof hemodynamic stability obtained from the heart sound signal. Thehemodynamic stability assessment module 430 deems the ventriculartachyarrhythmia to be unstable, such as when a measured change from thebaseline measurement of hemodynamic stability exceeds a predeterminedthreshold value.

FIG. 10 is a block diagram 1000 illustrating portions of a system thatincludes an IMD 1005 used to obtain patient hemodynamic information fromheart sounds. The IMD 1005 includes an implantable heart sound sensor1010, a heart sound sensor interface circuit 1015, and a controllercircuit 1025 that, in turn, includes a hemodynamic stability assessmentmodule 1030.

The IMD 1005 also includes a tachyarrhythmia detector circuit comprisinga tachyarrhythmia detection module 1020, a timer circuit 1035, and aventricular contraction sensing circuit 1045. In some examples, thetachyarrhythmia detector circuit also includes an atrial contractionsensing circuit 1040. The atrial contraction sensing circuit 1040provides a sensed atrial contraction signal. In the example shown, theatrial signal is sensed between lead tip electrode 1042 and lead ringelectrode 1043. The ventricular contraction sensing circuit 1045provides a sensed ventricular contraction signal. In the example shown,the ventricular signal is sensed between lead tip electrode 1047 andlead ring electrode 1048.

In some examples, the tachyarrhythmia detection module 1020 declaresthat a ventricular tachyarrhythmia has occurred based on the ventricularcontraction rate. For example, the tachyarrhythmia detection module 1020typically declares that a ventricular tachyarrhythmia has occurred whenthe ventricular contraction rate exceeds a threshold ventricularcontraction rate. In some examples, the tachyarrhythmia detection module1020 performs a rhythm discrimination method that includes recurrentlyupdating an average ventricular contraction interval (V-V interval). Incertain examples, the tachyarrhythmia detection module 1020 declaresthat a ventricular tachyarrhythmia has occurred when determining that anaverage ventricular contraction rate exceeds an average atrialcontraction rate by more than a specified rate threshold value. In someexamples, the tachyarrhythmia detection module 1020 declares that aventricular tachyarrhythmia has occurred based on a measure ofvariability of the ventricular contraction intervals.

If at least one episode of ventricular tachyarrhythmia in a subject isdetected, the hemodynamic stability assessment module 1030 determines ameasurement of hemodynamic stability of the ventricular tachyarrhythmia,such as from a heart sound signal. In some examples, the hemodynamicstability assessment module 1030 deems that a tachyarrhythmia isunstable based on: (1) a measurement of hemodynamic stability and (2) onan indication of unstable tachyarrhythmia received from thetachyarrhythmia detection module 1020. In some examples, such anindication of unstable tachyarrhythmia includes a measure of variabilityof the ventricular contraction intervals exceeding a predeterminedvariability threshold.

In an example, the hemodynamic stability module 1030 includes a baselinemodule to establish a baseline for the measurement of hemodynamicstability obtained from a heart sound signal. In an illustrativeexample, the measurement is the hemodynamic parameter He obtained fromthe S1 heart sound signal. In this example, the hemodynamic stabilityassessment module 1030 deems the ventricular tachyarrhythmia to beunstable when both a measured change from the baseline measurement ofhemodynamic stability exceeds a predetermined stability measurementthreshold value and the measure of variability of ventricular timeintervals exceeds a predetermined variability threshold value.

The IMD 1005 also typically includes a therapy circuit 1050. The therapycircuit 1050 typically includes a shock circuit to provide a high energyshock as a therapy for tachyarrhythmia. In some examples, the therapycircuit includes a switch network to electrically isolate senseamplifiers in the sensing circuits 1040, 1045 during delivery of theshock in order to prevent damage to the sense amplifiers. The controllercircuit 1025 can be configured to delay delivery of a predeterminedshock if the hemodynamic stability assessment module indicates thetachyarrhythmia is stable. In some examples, the measurement ofhemodynamic stability is the hemodynamic parameter He and the controllercircuit 1025 delays delivery of a shock if the measured He is below apredetermined threshold He value. In some examples, the controllercircuit 1025 delays delivery of a shock stimulus if the measured He isdifferent from a baseline He value by less than a predeterminedthreshold value.

In some examples, the therapy circuit 1050 includes ananti-tachyarrhythmia pacing (ATP) circuit coupled to the controllercircuit. The controller circuit 1025 initiates a predetermined ATPregimen upon detecting the episode of ventricular tachyarrhythmia andextends the duration of the ATP regimen according to the measurement ofhemodynamic stability. In some examples, the measurement of hemodynamicstability is the hemodynamic parameter He and the controller circuit1025 extends the duration of the ATP regimen if the measured He is belowa predetermined threshold He value. In some examples, the controllercircuit 1025 extends the duration of the ATP regimen if the measured Heis different from a baseline He value by less than a predeterminedthreshold value.

In some examples, the ventricular contraction sensing circuit 1045produces an electrical cardiac signal representative of cardiac activityof the patient. This cardiac signal is similar to the electrocardiogramsignal 625 in FIG. 6. In some examples, the IMD 1005 includes an A/Dcircuit to obtain digitized samples of the cardiac signal. The cardiacsignal is useful in identifying the different heart sounds. In someexamples, the controller circuit 1005 tracks maximum values of thedigitized samples of the cardiac signal to identify a QRS complex 630 asshown in FIG. 6. The QRS complex 630 can be used to identify the S1heart sound 610. The controller circuit 1025 in FIG. 10 then initiatesobtaining digitized samples of the heart sound signal provided by theheart sound interface circuit 1015 in correlation to the occurrence ofthe QRS complex. The digitized samples are obtained during a window ofpredetermined time duration. A measurement of hemodynamic stability isthen obtained from the digitized samples by the hemodynamic stabilityassessment module 1030.

Sensors can be included in cardiac function management (CFM) devices,such as to monitor a patient's activity. Indications of a patient'sactivity level are sometimes used to adjust a rate of pacing therapy ofa CFM device. Generally, these CFM devices increase a pacing rate inresponse to an increased activity level of the patient indicated by thesensor. This is sometimes referred to as rate responsive pacing. Anaccelerometer is one type of sensor that provides electrical signalsrepresentative of patient activity. It is sometimes preferable tomonitor heart sound signals while a patient is at rest. Thus, in someexamples, the hemodynamic stability assessment module 1030 is configuredto obtain the measurement of hemodynamic stability from a heart soundsignal in correlation with patient activity. Determining that a patientis at rest can be deduced from a patient's heart rate if the IMD 1005includes rate responsive pacing therapy—i.e. the patient's heart rate isat the resting heart rate when the patient is at rest.

In some examples, the IMD 1005 includes a communication circuit 1055 andthe controller circuit 1025 wirelessly communicates to the externaldevice 1060 information about at least one measurement of hemodynamicstability obtained from a heart sound signal. In some examples, theexternal device 1060 is an IMD programmer. In some examples, theexternal device 1060 is part of, or is in communication with, a computernetwork such as a hospital computer network or the internet. In someexamples, the external device 1060 is in communication with a mobiletelephone network. In some examples, the external device is a repeaterthat communicates wirelessly with the IMD 1005 and with a third devicein communication with a network, such as a computer network or mobiletelephone network. In some examples, the third device is an IMDprogrammer.

The controller circuit 1025 communicates an indication of a detectedtachyarrhythmia to the external device 1060. The indication can includeinformation that the tachyarrhythmia is stable or unstable. Theindication can be an entire digitized representation of a heart sound, adigitized representation of a cardiac signal, or an entire digitizedrepresentation of both a heart sound and a cardiac signal. The digitizedrepresentation can be displayed on the external device 1060 or anotherdevice connected to the network.

In some examples, the indication triggers an alarm on the externaldevice 1060. In some examples, an indication of an unstabletachyarrhythmia triggers the alarm. In some examples, the alarm includesa notification sent to a clinician or clinician's office over thecomputer network, such as by e-mail, or the alarm includes an indicationon a web page. In some examples, the alarm includes an indication ornotification sent to a medical device service center or an emergencyresponder. In some examples, the IMD 1005 includes a speaker and theindication of tachyarrhythmia is an audible alarm originating from theIMD 1005.

FIG. 11 is a block diagram of a method 1100 of monitoring mechanicalactivity of a heart. At 1105, at least one episode of ventriculartachyarrhythmia is detected in a subject. In some examples, an episodeof ventricular tachyarrhythmia is detected based on the ventricularcontraction rate. In some examples, an episode of ventriculartachyarrhythmia is detected when it is determined that an averageventricular contraction rate exceeds an average atrial contraction rateby more than a specified rate threshold value. In some examples, anepisode of ventricular tachyarrhythmia is detected based on a measure ofvariability of the ventricular contraction intervals.

At 1110, at least one heart sound signal for the subject is sensed usingan IMD. The heart sound signal is associated with mechanical vibrationof a heart of the subject. In some examples, the heart sound is the S1heart sound. In some examples, the heart sound signal is sensed with anaccelerometer and the heart sound signal is low-pass filtered to obtaina filtered heart sound signal. In some examples, higher frequencies ofthe filtered heart sound signal are pre-emphasized to add signal gain tothe higher frequency signal components.

In some examples, sensing a heart sound signal with an IMD includessensing an electrical cardiac signal that is representative of cardiacactivity of the patient. Heart sounds are identified, such as bycorrelating the sensed heart sound signal to the sensed electricalcardiac signal. For example, as shown in FIG. 6, if the electricalcardiac signal and the heart sound signal are correlated in time, thenidentifying a QRS complex in the sensed electrical cardiac signal 625helps identify the S1 heart sound 610. In some examples, digitizedsamples of the heart sound signal are obtained in correlation to theoccurrence of a feature of the electrical cardiac signal, such as theQRS complex. In some examples, the digitized samples are obtained duringa window of predetermined time duration. The window is defined relativeto an occurrence of a feature of the electrical cardiac signal.

Returning to FIG. 11, at 1115 a measurement of hemodynamic stability ofthe ventricular tachyarrhythmia from the heart sound signal is obtained.In some examples, obtaining a measurement of hemodynamic stabilityincludes extracting a proxy measurement of aortic pressure from theheart sound signal. In some examples, the heart sound signal includesthe S1 heart sound. In some examples, the heart sound signal includesother heart sounds.

In some examples, obtaining a measurement of hemodynamic stabilityincludes sampling the heart sound signal to obtain a plurality of signalsamples. In some examples, the signal samples are used to determine anautocorrelation function. In some examples, the measurement ofhemodynamic stability is the hemodynamic parameter He which iscalculated by any of the methods of calculating He discussed previously.

In some examples, a measurement of hemodynamic stability from the heartsound signal includes establishing a baseline measurement for themeasurement of hemodynamic stability. In some examples, establishing abaseline measurement includes determining a central tendency of themeasurement of hemodynamic stability. In some examples, the method 1100includes deeming the ventricular tachyarrhythmia to be unstable when ameasured change from the baseline measurement of hemodynamic stabilityexceeds a predetermined threshold value. In some examples, the method1100 includes deeming the ventricular tachyarrhythmia to be unstablewhen a measured change from the baseline measurement of hemodynamicstability exceeds a predetermined threshold value in X of Y consecutivecardiac cycles, wherein X and Y are integers and Y≧X.

It is sometimes preferable to monitor heart sounds while a patient is atrest. In some examples, sensing a heart sound signal with an IMDincludes sensing the signal in correlation to sensed patient activity toprovide an indication of when the patient is at rest. Information that apatient is or is not at rest can be used in the determination of stableor unstable tachyarrhythmia. For example, knowing a rest state of apatient may be used to decide whether to use X of Y consecutive cardiaccycles, or used to set the values of X and Y.

In some examples, the method 1100 includes deeming that a ventriculartachyarrhythmia is unstable based on a combination of a measurement ofhemodynamic stability from the heart sound signal and an indication ofunstable ventricular tachyarrhythmia obtained from an electrical cardiacsignal. In an example, the method 1100 includes establishing a baselinemeasurement for a measurement of hemodynamic stability from the heartsound signal and determining a measure of variability of ventriculartime intervals. The ventricular tachyarrhythmia is deemed to be unstablewhen both a measured change from the baseline measurement of hemodynamicstability exceeds a predetermined stability measurement threshold valueand the measure of variability of ventricular time intervals exceeds apredetermined variability threshold value.

In some examples, at 1120 the method 1100 includes delaying delivery ofa predetermined shock stimulus according to the measurement ofhemodynamic stability. If the measurement of hemodynamic stabilityindicates that the ventricular tachyarrhythmia is stable, the IMD maydelay providing such a stimulus. This gives the ventriculartachyarrhythmia time to spontaneously convert to a normal rhythm. Thismay reduce the number of shocks given to a patient, thereby improvingpatient comfort and extending the battery life of the IMD.

In some examples, the method 1100 includes initiating a predeterminedanti-tachyarrhythmia pacing (ATP) regimen upon detecting the episode ofventricular tachyarrhythmia. If the measurement of hemodynamic stabilityfrom the heart sound signal indicates that the tachyarrhythmia isstable, the duration of the ATP regimen is extended. This gives the ATPregimen time to convert the ventricular tachyarrhythmia to a normalrhythm. This also may reduce the number of shocks given to a patient.

FIG. 12 shows a block diagram of a method 1200 that extends an ATPtherapy regimen that is provided by an IMD. At 1205, the method includeschecking for ventricular tachyarrhythmia. At 1210, a regimen of ATPtherapy is selected and delivered. At 1215, a measurement of hemodynamicstability is obtained from a heart sound signal. At 1220, it isdetermined whether the ventricular tachyarrhythmia is stable using themeasurement of hemodynamic stability or using both a measurement ofhemodynamic stability and an indication of unstable ventriculartachyarrhythmia obtained from an electrical cardiac signal. Typically,the measurement of hemodynamic stability is obtained on a beat-by-beatbasis. If the ventricular tachyarrhythmia is stable, another cycle ofthe ATP regimen is delivered at 1225. If the ventricular tachyarrhythmiais indicated to be unstable, either a different ATP therapy regimen ischosen and delivered, or a shock stimulus is provided at 1230 to convertthe ventricular tachyarrhythmia to a normal sinus rhythm. At 1235 themethod ends if the ventricular tachyarrhythmia is converted to normalsinus rhythm, otherwise the IMD continues the ATP regimen whilemonitoring whether the ventricular tachyarrhythmia remains stable usingthe measurement of hemodynamic stability.

The systems and methods described herein use hemodynamic information,such as a heart sound proxy for blood pressure within the heartchambers, to improve the likelihood that the device will make a properassessment of heart rhythm stability. A proper assessment makes itpossible to delay the onset of treatment or to extend the time toattempt to resolve the episode with ATP before resorting to high-energyshock therapy. This results in a reduced number of necessary shockswhile ensuring that a high-energy shock stimulus will convert theabnormal rhythm if the rhythm fails to convert spontaneously or fails toconvert after ATP. Reducing the number of shocks given to a patientimproves patient comfort and extends the implantable life of the IMD byreducing the energy required for patient treatment.

The accompanying drawings that form a part hereof, show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments may beutilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations, or variations, or combinations of variousembodiments. Combinations of the above embodiments, and otherembodiments not specifically described herein, will be apparent to thoseof skill in the art upon reviewing the above description.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single embodiment for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted as reflecting anintention that the claimed embodiments require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive subject matter lies in less than all features of asingle disclosed embodiment. Thus the following claims are herebyincorporated into the Detailed Description, with each claim standing onits own.

1. A system comprising: an implantable medical device (IMD) comprising:an implantable heart sound sensor operable to produce an electricalsignal representative of at least one heart sound, the heart soundassociated with mechanical activity of a patient's heart; a heart soundsensor interface circuit coupled to the heart sound sensor to produce aheart sound signal; a tachyarrhythmia detector; and a controller circuitcoupled to the heart sound sensor interface circuit and thetachyarrhythmia detector, wherein the controller circuit includes ahemodynamic stability assessment module configured to: determine that atleast one episode of ventricular tachyarrhythmia is detected in asubject; initiate, in response to and obtain during the detected episodeof tachyarrhythmia, a measurement of hemodynamic stability of theventricular tachyarrhythmia from the heart sound signal, wherein themeasurement of hemodynamic stability is determined using linearprediction; and deem whether the ventricular tachyarrhythmia is stableaccording to the measurement of hemodynamic stability.
 2. The system ofclaim 1, wherein the hemodynamic stability assessment module includes abaseline module to establish a baseline for the measurement ofhemodynamic stability obtained from the heart sound signal; and whereinthe hemodynamic stability assessment module is further configured todeem the ventricular tachyarrhythmia to be unstable when a measuredchange from the baseline measurement of hemodynamic stability exceeds apredetermined threshold value.
 3. The system of claim 2, wherein thebaseline module is configured to establish a baseline for themeasurement of hemodynamic stability by determining a central tendencyof the measurement of hemodynamic stability.
 4. The system of claim 2,wherein the hemodynamic stability assessment module is configured todeem the ventricular tachyarrhythmia to be unstable when a measuredchange from the baseline measurement of hemodynamic stability exceeds apredetermined threshold value in X of Y consecutive cardiac cycles,wherein X and Y are integers and Y>X.
 5. The system of claim 1, whereinthe system further includes a timer circuit coupled to the controllercircuit to provide a ventricular time interval between ventricularcontractions; wherein the hemodynamic stability assessment moduleincludes a ventricular contraction variability module to obtain ameasure of variability of ventricular time intervals; wherein thehemodynamic stability module includes a baseline module to establish abaseline for the measurement of hemodynamic stability obtained from theheart sound signal; and wherein the hemodynamic stability assessmentmodule is further configured to deem the ventricular tachyarrhythmia tobe unstable when both a measured change from the baseline measurement ofhemodynamic stability exceeds a predetermined stability measurementthreshold value and the measure of variability of ventricular timeintervals exceeds a predetermined variability threshold value.
 6. Thesystem of claim 1, wherein the system further includes a shock circuitcoupled to the controller circuit, and wherein the controller circuit isconfigured to delay delivery of a predetermined shock stimulus duringthe detected episode of tachyarrhythmia according to the measurement ofhemodynamic stability obtained by the hemodynamic stability assessmentmodule.
 7. The system of claim 1, wherein the system further includes ananti-tachyarrhythmia pacing (ATP) circuit coupled to the controllercircuit, and wherein the controller circuit is configured to: initiate apredetermined ATP regimen upon detecting the episode of ventriculartachyarrhythmia; and extend a duration of the ATP regimen during thedetected episode of tachyarrhythmia according to the measurement ofhemodynamic stability.
 8. The system of claim 1, including at least oneimplantable cardiac signal sensing circuit coupled to the controllercircuit, the implantable cardiac signal sensing circuit configured toproduce an electrical cardiac signal representative of cardiac activityof the patient, and wherein the hemodynamic stability assessment moduleis configured to obtain the measurement of hemodynamic stability from aheart sound signal in correlation with the electrical cardiac activitysignal.
 9. The system of claim 1, including a patient activity sensorcoupled to the controller circuit, wherein the hemodynamic stabilityassessment module is configured to obtain the measurement of hemodynamicstability from a heart sound signal in correlation with patientactivity.
 10. The system of claim 1, wherein the heart sound sensorinterface circuit includes: a lowpass filter circuit; and apre-emphasizing circuit to add signal gain to higher frequencies of alowpass filtered heart sound signal.
 11. The system of claim 1,including an external device, and wherein the IMD includes acommunication circuit coupled to the controller circuit and the IMD isconfigured to communicate to an external device information about atleast one measurement of hemodynamic stability obtained from a heartsound signal.
 12. The system of claim 11, wherein the external device isan IMD programmer.
 13. The system of claim 11, wherein the externaldevice is a server in communication with a network, and wherein theserver is configured to communicate with the IMD via a third device. 14.The system of claim 11, wherein the external device generates an alarmbased on the information about the measurement of hemodynamic stability.15. The system of claim 1, including a timer circuit, coupled to thetachyarrhythmia detector, to provide a ventricular time interval betweenventricular contractions; and wherein the tachyarrhythmia detector isconfigured to declare that a tachyarrhythmia has occurred when anaverage ventricular contraction rate exceeds an average atrialcontraction rate by more than a specified rate threshold value.
 16. Thesystem of claim 1, wherein the measurement of hemodynamic stabilityincludes a hemodynamic parameter He, wherein He=1−k₁ ²,${k_{1} = {- \left( \frac{R_{1}}{R_{0}} \right)}},\mspace{14mu}{R_{0} = {\sum\limits_{n = 0}^{N - 1}\; s_{n}^{2}}},\mspace{11mu}{{{and}\mspace{14mu} R_{1}} = {\sum\limits_{n = 0}^{N - 2}\;{s_{n}s_{n + 1}}}},$where s_(n) are digitized samples of the heart sound signal obtainedduring a measured heart sound.
 17. The system of claim 1, wherein theheart sound sensor includes at least one of an accelerometer, a straingauge, and a microphone.